Sensors have long been used in the art to sense and measure a variety of environmental and/or physical states. Capacitive sensors have been particularly advantageous for having the capability to directly measure a variety of states, such as motion, chemical composition, electric field, etc., and, indirectly, sense many other variables that may be converted into motion or dielectric constants, such as pressure, acceleration, fluid level, fluid composition and the like. Additional applications for capacitive sensors include flow measurement, liquid level, spacing, scanned multiplate sensing, thickness measurement, ice detection, and shaft angle or linear position.
Generally speaking, during a typical design process for a capacitive sensor, electrode plates (or other surface types) use used to measure a desired variable. The capacitance for the plates is maximized by using the largest area allowed for the application, with the plates positioned in a close-space configuration. The sensor is preferably surrounded with appropriate guard or shield electrodes to handle stray capacitance and/or crosstalk from other circuits. Taking into consideration the sensor capacitance, stray capacitance and output signal swing, the sensor may be configure to operate according to a specified transfer function (area-linear, spacing-linear, etc.), and a plurality of balanced capacitors may be used for increased accuracy. The sensor may be further configured to operate at an excitation frequency high enough for low noise. As excitation frequency increases, external and circuit generated noise decreases.
FIG. 1 show an exemplary capacitive pressure sensor 100 as is known in the art. Sensor 100 comprises an upper housing 101 with an upper pressure port 103 and a lower housing 102 with a lower pressure port 102. Both the upper 101 and lower 102 housings may be constructed from stamping, casting, or machining a passivated metal such as stainless steel. The upper and lower housings are separated by a conductive diaphragm 109, and all of them are electrically coupled to a common reference, such as ground (110A-C). A sensing electrode 108 is configured to be positioned separately from diaphragm 109, and is separated therefrom by a small air gap.
During operation, as the relative pressure between the upper cavity and the lower cavity changes, the conductive diaphragm deflects to the side with lower pressure, resulting in a change in the gap between the sensing electrode and the conductive diaphragm. This change causes a change in capacitance between the sensing electrode and conductive diaphragm. By measuring this change in capacitance, the deflection of the diaphragm may be determined, indicating a relative pressure between the upper and lower cavities. Sensing electrode 108 is typically connected to a drive circuit 105 via electrical conductor 106. The electrical conductor 306 is typically shielded with an active shield 107 to protect from stray capacitance, which may comprise an in-phase signal buffered from the drive signal. Since the voltage differential between the electrode conductor 208 and the active shield 107 remain constant, there is no appreciable increase in measured capacitance.
Although the lower cavity 102 is separated from the sensing electrode 108 by a distance that is significantly greater than the distance that the sensing electrode 108 is separated from the conductive diaphragm 209, it has been found that the configuration of the lower cavity contributes parasitic capacitance to the sensing electrode 108. This parasitic capacitance becomes disadvantageous in that it forces a tradeoff between the size of the sensing electrode 108 and base capacitance. The size of the sensing electrode is important in that it affects capacitive change in the sensor, and consequently immunity from noise. Larger sensing electrodes will provide a greater change in capacitance with a given deflection of the conductive diagram. The larger capacitive change, as a result, will provide more noise immunity in the measurement. Accordingly, there is a need in the art to address these and other disadvantages in prior art capacitive sensors